Styrene-base plastics are widely used to fabricate housings and parts of household and office automation apparatus and equipments utilizing their excellent moldability, appearance and mechanical properties. For these applications, the fabricated articles are required to be flame retarded to avoid dangerous firing and flame-retarded styrene-based plastic compositions containing a halogenated flame retardant have been used to date. In addition to the flame retardancy, the plastic compositions are required to have increased heat stability, heat resistance, melt-flowability, impact strength and other properties for such applications.
A typical example of cases where fabricated plastic products are required to be self-extinguishable by melt-dripping is to produce plastic articles meeting grade V-2 in the vertical burning test method according to UL-94 standard of Underwriters Laboratoires. JP-B-5904360 and JP-B-62034784 disclose a flame retarding technology to achieve the above requirement by incorporating hexabromocyclododecane known to be effective in small amounts of addition into plastics. Hexabromocyclododecane is, however, thermally decomposed at a relatively low temperature. Therefore, when compounding with plastics under heat it decreases the heat stability of the resulting composition and changes color thereof from bright to dark.
German Patent No. 2,150,700 discloses bis(2,3-dibromopropyl) ether of tetrabromobisphenol A as a flame retardant having improved heat stability compared to hexabromocyclododecane. This flame retardant is still not satisfactory in heat stability and may produce scorching when used for instance, in the hot runner molding technique to shorten the molding cycle time and improve the efficiency of molding operations.
JP-A-05140389 discloses use of certain heat stabilizers in combination with bis(2,3-dibromopropyl) ether of tetrabromobisphenol A to remedy the above defect. The heat stability of the above flame retardant was found to be not entirely satisfactory in practice by this approach.
Use of another flame retardant in combination with tetrabromobisphenol A bis(2,3-dibromopropyl)ether has been proposed. JP-A-04239548 and JP-A-04292646 disclose a bromine-containing organic phosphorus compound and a bromine-containing epoxy polymer, respectively as a flame retardant to be combined. These proposals are proven, however, to be not entirely effective to increase the heat stability or deteriorative to the heat resistance, impact strength and other properties of plastics because a relatively large amount must be added.
JP-A-08208939 discloses a styrene-based plastic composition having well-balanced heat resistance, impact strength and heat stability containing a polyhalogenated diphenylalkane and an organopolysiloxane. Polyhalogenated diphenylalkane itself is, however, not uniformly dispersible in the plastics and the composition are often not stable in properties among different batches and also exhibit poor flowability.